The Tumor Microenvironment as a Drug Target, Part II

In the search for new targets, cancer researchers are taking a closer look at the cellular support networks that allow tumors to survive.!--h2>

Patricia Fitzpatrick Dimond, Ph.D.

Investigators are emphasizing the key role of the cancer cell stroma during the progression and maintenance of multiple cancers, particularly solid epithelial tumors including breast cancers. And they note that although cancer drug development has historically focused on targeting tumor cells, “emphasis has shifted toward the tumor microenvironment (TME) for novel therapeutic and prevention strategies.”

Cancer-associated fibroblasts (CAFs) comprise most of the bulk of cancer stroma, affecting the tumor microenvironment so they promote cancer initiation, angiogenesis, invasion, and metastasis. In breast cancer, CAFs not only promote tumor progression but also induce therapeutic resistance. Continuous infiltration of a tumor mass by various fibroblast progenitors and their subsequent transdifferentiation into CAFs can facilitate the generation and expansion of the so-called desmoplastic, reactive stroma that in turn supports tumor progression.

Walking the C-Path

The importance of the stroma in influencing the development of cancer was underscored in 2009, as scientists at the Stanford University School of Engineering and pathologists at Stanford School of Medicine reported they had developed computer models to analyze breast cancer microscopic images. The model, called Computational Pathologist or C-Path, provides a machine-learning-based method for automatically analyzing images of cancerous tissues and predicting patient survival.

Pathologists have used three specific features for evaluating breast cancer cells: the percentage of the tumor comprised of tube-like cells, the diversity of the nuclei in the outermost (epithelial) cells of the tumor, and the frequency with which those cells undergo mitosis. These three factors are studied microscopically and scored qualitatively to stratify breast cancer patients into three groups that predict survival rates.

However, “tumors contain innumerable additional features, whose clinical significance has not previously been evaluated,” said Andrew Beck, M.D., a doctoral candidate in biomedical informatics and the paper’s first author.

C-Path can assess 6,642 cellular factors. Once “trained” using one group of patients, C-Path was asked to evaluate tissues of cancer patients it had not checked before and the result was compared against known data. Ultimately, C-Path yielded results that were statistically significant in predicting patient survival. Moreover, the prognostic model built on just the three stromal features was a stronger predictor of patient outcome than one built from the eight top epithelial features.

“We built a model based on features of the stroma—the microenvironment between cancer cells—that was a stronger predictor of outcome than one built exclusively from features of epithelial cells,” said Beck. “The stromal model was as predictive as the model built from both stromal and epithelial features.”

RhoB and AKT Signaling

Investigators at Beth Israel Deaconess Medical Center and Harvard Medical School observed that the distinct characteristics of stromal cell signaling networks are not usually considered in developing tumor-targeted drugs—an oversight, the authors say, that potentially confounds proof-of-concept studies and increases drug development risks.

Investigators examined, in established murine and human models of breast cancer, how differential regulation of Akt by the small GTPase RhoB in cancer cells or stromal endothelial cells determines their dormancy versus outgrowth when angiogenesis becomes critical. Rho-family GTPases are molecular switches that transmit extracellular cues to intracellular signaling pathways.

In in vivo or in vitro cancer cells, RhoB acts as a tumor suppressor, restricting epidermal growth factor receptor (EGFR) on the cell surface as well as Akt signaling.

But following activation of the angiogenic switch, RhoB functions as a tumor promoter by sustaining endothelial Akt signaling, growth, and survival of stromal endothelial cells that mediate tumor neoangiogenesis. The term “angiogenic switch” refers to a time-restricted event during tumor progression where the balance between pro- and anti-angiogenic factors tilts towards a pro-angiogenic outcome. This process results in the transition from dormant avascularized hyperplasia to outgrowing vascularized tumor and eventually to malignant tumor progression.

Altogether, the positive impact of RhoB on angiogenesis and progression dominates its negative impact in cancer cells themselves, the investigators said. The authors conclude that their results elucidate the dominant positive role of RhoB in cancer and show how differential gene function effects on signaling pathways in the tumor stromal component can complicate the challenge of developing therapeutics to target cancer pathophysiology.

More on CAFs and Breast Cancer

Scientists at Perelman School of Medicine of the University of Pennsylvania and the Wistar Institute reported that breast cancer subtype-specific changes occur in CAFs derived from breast cancer. Lead author Julia Tchou, M.D., Ph.D. of Penn and Wistar colleagues asked whether the reported gene expression profile differences between CAFs in breast cancer stroma and normal breast fibroblasts could be stratified based on tumor subtypes.

The investigators observed significant expression differences between CAFs derived from Her2+ breast cancer and CAFs from TNBC and ER+ cancers, particularly in pathways associated with cytoskeleton and integrin signaling.

To explore whether CAFs derived from various breast cancer subtypes can differentially enhance the migratory phenotype of breast cancer cells, the researchers compared the migration of breast cancer cells cultured in the presence or absence of CAFs isolated from ER+, Her2+, and TNBC cells. As their expression profile results predicted, CAFs derived from Her2+ breast cancer significantly enhanced the migration of T47D.

In the case of Her2+ breast cancer, the signaling pathways found to be selectively up regulated in CAFs likely contribute to the enhanced migration of breast cancer cells in transwell assays and may contribute to the unfavorable prognosis of Her2+ breast cancer, they said.

And as noted in the first part of this two part series focusing on cancer stromal drug targets, one potential stromal drug target is FAP (fibroblast activation protein), a cell surface serine protease selectively expressed on tumor-associated fibroblasts and other mesenchymal stromal cells in epithelial tumors. Dr. Tchou told GEN that FAP represents an exciting potential target in breast cancer. “Existing and future FAP directed therapy may be explored in conjunction with conventional chemotherapy to augment efficacy of existing therapy against breast cancer,” she said.

miRNAs and Metastasis

And studies analyzing the signaling pathway dynamics and cellular interactions that occur during the development and treatment of metastatic disease have also suggested novel stromal tumor targets.

Noting the active participation of the tumor stroma in cancer progression investigators at the department of pharmacology and cancer biology at Duke University Medical Center identified mechanisms elucidating the role of microRNAs in regulating cancer metastasis.

These miRNAs regulate metastasis by modulating the intrinsic properties of tumor cells and their interaction with the tumor stroma. The investigators showed that both strands of the miR-126/miR-126* duplex simultaneously target the stromal cell-derived factor-1 alpha (SDF-1α) cytokine, thereby reducing the recruitment of mesenchymal stem cells and inflammatory monocytes into the tumor stroma of primary tumors, and inhibiting lung metastasis.

The investigators reported that the miRNA pair inhibited lung metastasis by breast tumor cells in a mouse xenograft model through direct inhibition of SDF-1α expression, and indirectly, through the suppression of the expression of chemokine (C-C motif) ligand 2 (Ccl2) by cancer cells in an SDF-1α-dependent manner.

Expression of this miRNA pair is downregulated in cancer cells by promoter methylation of their host gene Egfl7. These findings, the scientists concluded, determined how this microRNA pair alters the composition of the primary tumor microenvironment to favor breast cancer metastasis, and demonstrates a correlation between miR-126/126* downregulation and poor metastasis-free survival of breast cancer patients.

And while drugs that target cancer stroma components may remain a ways off, searching for targets continues to lead to better understanding of tumors as organ systems, capable of creating their own ecosystems that sustain their survival and metastasis.